/prog/ challenge

Write a macro that, given a term of code, transforms all the calls to the function FOO in that term, and only those calls, into calls to function BAR with the same arguments.
Language: Common Lisp or Scheme.

Why is
(qux `(let ((foo 1)) (+ 3 foo)))

different from

(defparameter *anus `(let ((foo 1)) (+ 3 foo)))

(qux *anus)
?

Lisp is like an ultimate manifestation of referential opaqueness and shittiness.